The present invention relates in general to communication systems, and is particularly directed to an amplitude modulation (AM) based mechanism for controllably altering the shape of a binary phase shift keyed (BPSK) digital spreading waveform to be modulated onto an RF carrier and amplified by a saturated RF amplifier. The spectral properties of an RF carrier modulated with the AM-BPSK waveform and amplified by a saturated RF amplifier, when combined with the spectral properties of an RF carrier modulated with the BPSK waveform and amplified by a saturated RF amplifier, produce a composite AM-BPSK/BPSK modulated RF waveform containing substantially suppressed sidelobes.
The need for increased capacity accompanying the expansion of the wireless communications market has resulted in a move away from traditional analog modulation techniques, such as frequency modulation (FM), to digital modulation formats, such as time division multiple access (TDMA), code division multiple access (CDMA) and direct spread spectrum (DSS) waveforms. Although reasonably priced RF output amplifiers are capable of providing the relatively high degree of linearity such waveforms require, they do so at a low and practically unacceptable power usage efficiency (e.g., on the order of only fifteen percent). To realize the more acceptable efficiencies (e.g., on the order of 35-75%) that are obtainable from such amplifiers, it is necessary to operate the amplifiers in their saturation regions. Unfortunately, the non-linear distortion associated with operating the RF amplifier at saturation causes spectral regrowthxe2x80x94pushing a non-insignificant amount of amplified energy outside a prescribed relatively narrow bandwidth (such as that mandated by FCC requirements).
Conventional approaches to solve this problem have included both pre- and post-amplifier filters. Pre-amplification filtering only affects the input to the amplifier; spectral regrowth still occurs, forcing the use of a non-saturated amplifier. A post filter, on the other hand, serves to remove spectral energy outside the intended bandwidth. Unfortunately, this has the undesired effect of introducing loss after the amplifier, which impacts overall transmitter efficiency, and thereby mitigates against the reason for operating the amplifier at saturation in the first place.
In accordance with the present invention, the above-described spectral regrowth problem is successfully addressed by controllably altering the shape of (amplitude modulating (AM)) a (binary phase shift keyed (BPSK) digital spreading) waveform to be modulated onto an RF carrier and amplified by a saturated RF amplifier. The spectral properties of the RF carrier when modulated with the AM-BPSK waveform and amplified by a saturated RF amplifier are different from the spectral properties of a similarly amplified RF carrier modulated with the BPSK waveform that has not been subjected to the amplitude modulation. This difference is such that, when the two modulated and amplified waveforms are combined, the spectral properties of the resulting composite modulated and amplified RF carrier contain substantially suppressed sidelobes.
For this purpose, pursuant to a non-limiting embodiment of the invention, a digital spreading sequence to be modulated onto a transmitted RF carrier is converted into a BPSK waveform. The BPSK waveform is applied to first and second modulation paths of an RF carrier modulator. One of the paths through the RF carrier modulator is coupled directly to a first RF carrier mixer, while a second path through the modulator is coupled to an amplitude modulating switch, that is installed upstream of a second RF carrier mixer. The amplitude modulating switch is operative to modulate the amplitude of the BPSK waveform in accordance with an amplitude modulating or xe2x80x98choppingxe2x80x99 signal. This chopping signal may be readily generated by processing the digital spreading sequence and its associated clock, so that the chopping signal opens the amplitude modulating switch for a prescribed intervalxe2x80x94beginning prior to and concluding after each transition in a delayed version of the BPSK waveform. This has the effect of selectively reducing portions of the amplitude of the BPSK waveform to zero on either side of its phase transitions between 0xc2x0/180xc2x0 and between 180xc2x0/0xc2x0.
The BPSK waveform in the first path, and the amplitude modulated (AM)-BPSK waveform produced by the amplitude modulating switch in the second path, are modulated onto an RF carrier by their respective mixers and then amplified in associated RF amplifiers, each operating at saturation. The outputs of the two saturated RF amplifiers are summed together to produce a composite signal that is transmitted. Because of the xe2x80x98choppedxe2x80x99 reductions in the amplitude of the AM-BPSK waveform on either side of relatively xe2x80x98steepxe2x80x99 transitions in the original BPSK signal, the AM-BPSK waveform has xe2x80x98steppedxe2x80x99 data transitions, that are effective to shift or modify its spectral properties relative to those of the original BPSK signal, which is not so chopped.
This causes a decrease in the total power during phase changes of a prescribed portion (e.g., one-half) of a chip. Spectral analysis reveals that the energy in sidelobes of the spectrum of the composite or combined (BPSK+AM-BPSK) amplified waveforms tends to be shifted or concentrated within the intended limited bandwidth of RF amplifier operation. In addition, the depths of the notches of the sidelobes of the composite AM-BPSK/BPSK spectrum are significantly suppressed. In contrast, most of the energy in the sidelobes of the unmodulated BPSK waveform per se, and in sidelobes of the AM-BPSK waveform falls outside this limited bandwidth.